164 THE MAIN CURRENTS OF ZOOLOGY 



thoroughly entrenched in the mind of the scientific 

 world. Never before was it so thoroughly supported 

 by such a wealth of compelling evidences which have 

 multiplied with the progress of biological investiga- 

 tion. The scientific discussions are no longer re- 

 garding the occurrence of evolution. The fact of 

 evolution is so widely recognized that it is regarded 

 in the light of a great truth of nature. But, regarding 

 the factors the particular agencies that have been 

 at work in nature to bring about this recognized 

 development of life there is much room for discus- 

 sion. The attempt to designate the particular factor, 

 or factors, has given rise to the different theories of 

 organic evolution. That natural selection is an 

 important agent at some stages of the process is 

 commonly conceded, but the factors more primary 

 in nature which produce variation and adaptation 

 are more obscure and will for a long time be subjects 

 of investigation. 



